1. Field of the Invention
This invention relates to a false contour reduction device, display device, false contour reduction method, and false contour reduction program.
2. Description of the Related Art
A display device receives an input image signal and displays an image in pixel units. As shown in FIG. 12A of the accompanying drawings, a display device sequentially scans the pixels in the following order: pixel Pi−4→pixel Pi−3→pixel Pi−2→pixel Pi−1→pixel Pi→pixel Pi+1→pixel Pi+2→pixel Pi+3, to display the image.
The display device expands an input image signal into a time series of subfields having different gradation levels (with different brightness weightings), that is, into a plurality of subfields with different emission durations, and selects a combination of light-emitting subfields within a single field of image according to the intended gradation, so that the display device can perform appropriate gradation representation of each pixel.
In FIG. 12A, one field is divided into eight subfields, from the first subfield to the eighth subfield.
When the display device displays a moving image, or when the display device displays a still image but an observer moves his line of vision as indicated by the arrow in FIG. 12A, unexpected gradation changes are perceived, as indicated in FIG. 12B. That is, false contours are observed.
This is because emission in each subfield is performed in order in a time series and therefore the image of each subfield is perceived as an afterimage by an observer, undifferentiated from the subfields of adjacent gradations in the time series and from the subfields of the next image field.
Such false contours occur especially prominently in places where the weighting of the subfields changes significantly.
An explanation of false contours is given in, for example, a book entitled “All About Plasma Display” Hiraki Uchiike and Shigeo Mikoshiba, Kabushiki Kaisha Kogyo Chosakai, May 1, 1997, pp. 164-165.
At places where false contours occur, the observer does not perceive the gradations originally intended. In the case of grayscales, the observer perceives this as gradation inversion. In the case of color display, the observer perceives completely different colors. Hence an image is experienced which is markedly degraded compared with the input image.
Thus, it is essential that false contours be reduced in display devices such as plasma displays which employ a subfield emission scheme for gradation representation.